Laser Hazard Evaluation

A. Laser Environmental Factors:
Three aspects of the application of a laser or laser system influence the total hazard evaluation:

The laser or laser system’s ability to injure personnel

The environment in which the laser is used

The personnel who may use or be exposed to the beam

All three aspects must be considered in order to establish control measures commensurate with the potential hazard.

The environment in which the laser is used may vary with each application. It is extremely important, however, that the environment in which the laser is used be considered in order to determine whether or not the control measures in are adequate, or if some are unnecessary. For example, the controls for a laser robotic system used on a production floor would be expected to be considerably different from those used in a research laboratory. As a minimum, the following shall be considered:

Number of lasers or laser systems

Degree of isolation (laboratory, production floor)

Probability of the presence of uninformed, unprotected transient personnel

Permanence of beam path(s)

Permanence of specularly reflecting objects in or near the beam path

The use of optics (e.g., lenses, microscopes, optical fibers)

B. Laser Safety Officer (LSO):
The conditions under which the laser is used, the level of safety training of individuals using the laser and other environmental and personnel factors are important considerations in determining the full extent of safety requirements. Since such situations require informed judgments by responsible persons, major responsibility for such judgments has been assigned to a person with the requisite authority and responsibility, namely the Laser Safety Officer (LSO).

The LSO shall have the authority and responsibility to monitor and enforce the control of laser hazards, and to effect the knowledgeable evaluation and control of laser hazards. This shall be done at each location or administrative area where Class III or Class IV lasers or laser systems are used or manufactured.

Designation of an LSO is generally not required for operation of a Class II or Class IIIA laser or laser system. Designation of an LSO is generally not required if maintenance and service are limited to Class I and Class II laser systems which do not contain embedded lasers of a Class higher than Class II. If service is performed on a laser product having an embedded Class IIIA, Class IIIB, or Class IV laser, there shall be a designated LSO.

Depending on the number and classification of lasers and laser systems, within a location or administrative area, the position of LSO may not be a fulltime assignment.

C. Standard Operating Procedure:
One of the most important, but often least used, control measure is the requirement to develop a written Standard Operating Procedure (SOP). The key to an effective SOP is the participation, during its preparation, of all individuals (including the LSO) that will operate, maintain, monitor, and/or service the equipment. A good starting point for an SOP would be the instructions for safe operation suggested by the manufacturer; however these may not always be appropriate for a specific application due to special use conditions.

An SOP is considered as an administrative/procedural control and is required for all Class IV lasers and laser systems. An SOP is recommended for Class IIIB lasers, especially those CW lasers operating above 200 mW in an open configuration.

D. Laser Personnel:
The personnel who may be in the vicinity of a laser and its emitted beam(s) and the operator can influence the total hazard evaluation. Hence, they can influence the decision to adopt additional control measures not specifically required for the class of laser being employed. The type of personnel influences the total hazard evaluation. It must be kept in mind that for certain lasers or laser systems (for example, some Class IIIA lasers used for alignment tasks), the principal hazard control rests with the operator; that it is his or her responsibility not to aim the laser at personnel or flat mirrorlike surfaces. If individuals unable to read or understand warning labels are exposed to potentially hazardous laser radiation, the evaluation of the hazard is affected and control measures may require appropriate modification.

The following are considerations regarding operating personnel and those who may be exposed:

Maturity of judgment of the laser user(s).

General level of training and experience of the laser user(s), (that is, whether part time employees, scientists, etc.).

Awareness of onlookers that potentially hazardous laser radiation may be present, and of relevant safety precautions.

Degree of training in laser safety of all individuals involved in the laser operation.

Reliability of individuals to follow control procedures.

Number and location of individuals relative to the primary beam or reflections, and the potential for accidental exposure.

Other hazards not due to laser radiation which may cause the individuals to react unexpectedly, or which influence the choice of personnel protective equipment.

E. The Nominal Hazard Zone:
The Nominal Hazard Zone (NHZ) associated with Class IIIB and Class IV lasers shall also be determined. The NHZ describes the space within which the level of direct, reflected, or scattered radiation during normal operation exceeds the appropriate MPE’s and is determined from the following characteristics of the laser:

Power or energy output

Beam diameter

Beam divergence

Pulse repetition frequency (prf)

Wavelength

Beam path including reflections

Beam profile

Maximum anticipated exposure duration

NOTE: Examples of NHZ calculations are given in the appendix of ANSI Z136.1 (1986). In addition, computer software is also available to assist in the computations for NHZ, protective eyewear optical densities and other aspects of laser hazard analysis.

It is often necessary in some applications where open beams are required (vis: industrial processing, laser robotics) to define the area where the possibility exists for potentially hazardous exposure. This is done by determining the Nominal Hazard Zone (NHZ) which is, by definition, described by the space within which the level of direct, reflected or scattered radiation exceeds the level of the applicable MPE. Consequently, persons outside the NHZ boundary would be exposed below the MPE level and are considered to be in a “safe” location. The NHZ boundary may be defined by direct (intrabeam) beams, diffusely scattered laser beams as-well-as beams transmitted from fiber optics and/or through lens trains… etc. In other words, the NHZ perimeter is the envelope of MPE exposure levels from any specific laser installation geometry.

The purpose of an NHZ evaluation is to define that region where control measures are required. Thus, as the scope of laser uses has expanded, the classic method of controlling lasers by enclosing them in an interlocked room has become limiting and, in many instances, can be an expensive over-reaction to the real hazards present.

1. Intrabeam Nominal Hazard Zone:
If the value of the irradiance at a distance (R) away from the laser is maintained at (or below) the MPE, then the distance is considered the intrabeam NHZ range (RI.B. NHZ) or “safe range” value.

For example, consider the case of a 300 watt Class IV (open beam) industrial Nd:YAG laser materials processing system with a beam divergence of 2.5 milliradian and an exit beam diameter of 0.4cm. Using EQUATION 1 and assuming the long term (8 hour) “worst case” MPE of 1.6 mW/cm(2), we find that one would have to be nearly two kilometers away from this laser before the beam would spread to a size large enough that it would reduce the laser beam irradiance to the MPE level of 1.6 x 10(-3) W/cm(2). This distance is certainly larger than an industrial facility, hence controls would be needed to both confine the hazard and protect those in the space.

2. Diffuse Reflections:
In practice, most slightly rough non-glossary surfaces act as diffusing surfaces to incident laser beams. A diffusing “rough” surface acts as a plane of very small scattering sites that reflect the beam in a radially symmetric manner. The roughness of the surface is such that the scattering sites are larger than the laser wavelength. Consequently the reflected radiant intensity expressed as power per unit solid angle (W/sr), denoted by I(theta), is dependent upon incident intensity (I(o)) and the cosine of the viewing angle (theta) (both measured from the normal to the surface) by LAMBERT’S COSINE LAW and a surface behaving in this manner is generally referred to as a Lambertian surface. This relationship DEFINES an ideal plane diffuse reflector.

It should be stressed that “rough” surfaces do not always act as diffuse reflectors at ALL WAVELENGTHS. For example, brushed aluminum (which is partially diffuse for visible wavelength laser radiation) is a good specular (mirror-like) reflector for far-infrared wavelength lasers such as the CO(2) laser (10.6 µm). However, if the metal surface is melting (such as during a laser welding process) the laser beam back reflected from the weld puddle will usually obey a cosine scattering relationship.

Additionally, most slightly “rough” surfaces may still have some properties that also contribute some specular reflection component. This may occur with just a few percent of the incident radiation specularly reflected and the remainder diffusely reflected. This behavior is generally the rule, and not the exception, for most common surfaces. As a result, a constant power distribution of the reflected radiation is not exactly radially symmetric, but will skews toward the specularly reflected component.

A laser beam reflected from a diffuser is often expressed in radiant energy units which combine the reflected radiant power (or energy) with the geometry of a solid angle “cone” and the reflected “source” area.

For example, let’s assume that a 1 mW HeNe “aiming laser” beam is directed a distance of 10 meters across the room onto a 100% diffusely reflecting wall. The irradiance on the wall will be 1.1 mW/cm(2). Assuming the reflectivity of the surface to be 100% (rho = 1.0), we find from Equation 3 that the radiance of the reflected beam L = 0.35 x 10(-)3 W/cm(2)sr.

For comparative purposes, consider that staring directly at a standard 100 watt frosted light bulb at close range is equivalent to viewing a diffuse light source with a radiance of about 40 mW/cm(2)sr. Hence the diffuse reflection of a 1 mW HeNe laser directed onto a wall 10 meters away is over 100 times less “bright” than viewing a 100 watt diffused light bulb! Hence diffuse viewing of low power laser light can offer no more hazard (and maybe less) than more conventional light sources. The dividing point between hazardous and non hazardous diffuse reflections with cw lasers is generally considered to be 0.5 watt (the dividing point between cw Class IIIB and Class IV lasers).

3. Inverse Square Law:
The reflected irradiance (E) or radiant exposure (H) from a Lambertian surface at some distant point is inversely related to the square of the distance (r) from the surface. This describes diffuse reflections from a point source.

The inverse square relationship with distance holds as long as the distance (r) is much greater than the spot diameter D(L). Consequently, a diffuse surface acts as a distance-dependent attenuator that permits indirect viewing of some low powered laser beams when the reflecting spot is small. Obviously, if the laser power is sufficient (ie: 0.5 watts), even a diffuse reflection is hazardous to view. This is an important consideration for those working with high powered visible or near infrared Class IV lasers where specific control methods are required for safe use.

4. Diffuse Reflection Nominal Hazard Zone:
There are some instances where it is useful to calculate the distance away from a “point source” diffuse reflector at which a specific irradiance occurs. Solving the inverse square law for distance, we find that the diffuse reflection nominal hazard zone (R(D.R.NHZ)) can be written.

For example, assume 45 degree (cos theta = cos (450) = 0.707) viewing of a 50 W xenon fluoride excimer UV laser directed onto a surface with a 75% reflectance at the 0.351 µm wavelength. At what distance does the long term (3×10(4) sec.) MPE irradiance of 33.3 µW/cm(2) occur? Calculations show that one needs to be over five meters away for a “safe” exposure to the backscattered UV excimer laser beam in this example. Similarly, it can be shown that the maximum NHZ range for a 100% diffuse reflection from a 300 watt Nd:YAG laser (MPE=1.6×10(-3) W/cm(2)) will be 244 cm or about 8 feet!

5. Extended Source Diffuse Reflections:
In cases where the laser creates large sized spots on the diffuse target (relative to the viewing distance), the diffuse surface is said to create an “extended source” relative to the eye. In this case, the retinal image size of the focused laser light will usually exceed 100 µm and the viewer can resolve the details of the diffuse target source. Such larger area retinal images are of special concern because the threshold for biologic damage for the larger retinal images is at least TEN TIMES LOWER than for point source images.

Also significant is the fact that in this case the resulting retinal irradiance produced while viewing an extended source can be shown to be INDEPENDENT of the distance between the source and viewer. Therefore, as one moves away from the source, the focused retinal spot becomes smaller but the retinal irradiance remains constant. In general, the condition applies up to that point where the source can still be resolved by the viewer. Beyond that point, the retinal image size no longer changes with distance and the point source diffuse relationships apply.

In practice, the evaluation of the point source/extended source dilemma has been addressed in the ANSI Z-136 standard by requiring an evaluation of the Angle subtense angle (alpha) between the viewer and the extended source target. For Lambertian (diffuse) viewing, this angle is also a measure of the resultant retinal image size (dr = f alpha); where f is the focal length of the eye, approximately 17 mm.

The cut-off between point source and extended source occurs at the “minimum” viewing angle, called alpha (MIN), which corresponds to the MAXIMUM viewing distance (R(MAX)) for which extended source MPE values apply.

For example, the ANSI Z-136 standard indicates that in the time frame from 10(3) to 3×10(4) seconds, the extended source MPE for visible and near infrared frequencies is given by the following expression for the radiance (L(p)): MPE = 0.64 x C(A) (W/cm(2)/sr) Where C(A) = 5 is the near infrared correction factor in the wavelength range from 1.051 to 1.400 µm.

Assume, for example, a CW 300 watt Nd:YAG laser is directed onto a 100% diffusely reflecting wall through a short focal length lens so-as-to produce a spot diameter on the wall of 10 inches (DL = 25.4 cm). The ANSI Z-136.1 standard indicates that an minimum of 24 milliradians applies. Thus, the applicable MPE can be determined from the equation above. Substituting and using the value of C(A) = 5.0, the extended source MPE = (0.64) x (5) = 3.2 W/cm(2)/sr.

A similar computation can show that the reflected radiance will just reach the MPE value when the spot diameter is reduced to 6.2 cm. In this case, the extended source condition would apply for a distance up to 2.6 meters normal to the reflecting surface. Spot sizes less than 6.2 cm will produce an extended source viewing hazard region (L MPE) which will extend out into 2 pi steradian zone surrounding the reflecting point. This is a very serious viewing condition since the viewing angle can be from anywhere in the area around the reflecting point. In addition, a very large retinal image is produced which can result in a large retinal damage area.

6.Lens on the Laser Nominal Hazard Zone:
Most industrial laser uses incorporate a lens as the final component in the beam path. This not only provides the increased irradiance in the focal plane of the lens to do the work intended of the laser, but it also causes the beam to spread with an angle usually many times larger than the inherent laser beam divergence in the space beyond the focal plane. Consequently, the MPE irradiance is reached in a distance much less than the intrabeam NHZ.

For example, consider a 3000 watt CO(2) laser with a 5 inch focal length lens in place. Assume the beam size at the lens is 1 inch.

Thus, in the direction defined by the cone of laser light directed through the lens, the hazard zone extends up to a distance of 9.8 meters, at which point the beam has expanded in diameter.

7. Fiber Optic on Laser Nominal Hazard Zone:
In a manner similar to the lens-on-laser condition, a fiber optic attached in the beam path also provides a beam expanding element that shrinks the hazard range depending upon the characteristics of the fiber. For a typical multimode fiber used for some industrial Nd:YAG applications, with a numerical aperture: NA = 0.20 attached to a 300 watt Nd:YAG laser, the nominal hazard zone range is roughly equivalent the hazard range for a 300 watt Nd:YAG laser system with a 3.5 mm beam size and a 15 mm focal length lens in the beam path. This is reasonable since a fiber optic is optically equivalent to a short focus lens in the beam path.

F. Intrabeam Optical Density Determination:
Based upon these typical exposure conditions, the optical density required for suitable filtration can be determined. Based upon the worst case exposure conditions outlined above, one can determine the optical density recommended to provide adequate eye protection for this laser. For example, the minimum optical density at the 1.06 µm Nd:YAG laser wavelength for a 10 second direct intrabeam exposure to the 100 watt maximum laser output can be determined as follows:

An extremely conservative approach would be to choose an 8 hour (occupational) exposure. In this case, the optical density at 1.06 µm is increased to OD = 5.2 for a 100 watt intrabeam exposure because the 8-hour (30,000 seconds) MPE is reduced to 1.6 x10(-3) W/cm(2).

G. Surgical Fiber OD Hazard Analysis:
A hazard analysis of a typical Nd:YAG surgical laser with a fiber optic hand-piece attachment could be based upon the following parameters:

Laser power: 100 Watts (maximum/CW)

Beam divergence: 210 milliradian (12 degrees from fiber tip)

Exposure time: 10 seconds (maximum); Wavelength: 1.06 µm

Using these parameters, a mathematical hazard analysis can be done to estimate the general region around the surgical site where hazardous exposures may be possible. Although, the following analysis is based upon one specific unit, it is representative of Nd:YAG surgical lasers. This analysis is based upon the maximum permissible exposure (MPE) criteria of the ANSI Z-136.1 standard.

The “worst case” MPE value for a direct intrabeam Nd:YAG laser exposure of 10 seconds is 50.6 millijoules/cm(2). The MPE for a 10 second diffuse reflection of this laser is 10(8) Joules/cm(2) sr. contained within an apparent visual angle (alpha min) which is not smaller than 24 milliradians. The 10 second MPE value for skin exposure is 10.5 Joules/cm(2).

To estimate a diffuse reflection from the site, one can estimate, using the inverse square law, an approximate scattering distance of 40 cm from the beam (on the tissues) to the eye. Using the ANSI Z-136.1 point source criteria (because the focused beam acts as a point source), the irradiance at the eye will be 19.9 mW/cm(2). This produces a radiant exposure of nearly 200 mJ/cm(2) during a ten second exposure.

The optical density required for safe viewing of the diffuse reflection off tissues is substantially reduced from the 100 watt intrabeam case. Using a 40 cm “viewing distance”, and assuming a “point source condition, the required optical density at 1.06 µm would be OD = 0.6 for a 10 second exposure and OD = 1.1 for an 8 hour (occupational) exposure.

The “worst case” conditions suggest than an optical density ranging from 0.6 to 5.2 depending upon viewing time and conditions.